heat treating 4140 prehard?

Timy

Well-Known Member
Would it be possible to get some 4140/4142 that is prehard around 30-40rc and just heat treat the edge of a chopper I am making with a torch to non magnetic then quench in oil? Also what tempering range would I be looking at if I was shooting for 56-57? I promise I have looked but the info I can find is vague, along the lines of temper between 400 and 1200. Thats it.

Basically I want to keep the majority of the tool in the 30-40 range, but the edge at 57ish.

Thanks for your time ya'll.
 
Either one of those steels will be marginal at forming martensite and my reference for 4140 list it's as quenched hardness as about 55-56 HRc. The chromium and molybdenum does push the nose of the cooling curve to the right so an oil quench should give you no problem. I think that I would be sure to give it a 2-3 minute soak to make sure I've gotten all the carbon into solution and then quench. For tempering I would start at 325° and then test the edge and adjust from there. Do your heating of the blade in a darkened area of after sunset to interpret the color of the steel right and aim for whatever color is supposed to be ~1500°. You will only need one two hour tempering cycle. There's not enough carbon in those steels to make retained austinite a problem.

Doug
 
I agree with Doug.....MARGINAL is being generous..... with 4140/42 what will happen is there's simply not enough carbon, to create the necessary hardness to support a sharp edge. It will sharpen easily, but the first thing you cut will dull it to a "butter knife" edge......and the first thing you try to chop (short of a block of jell-o) :) will "dent" the edge. Just not a good choice for what you're trying to do. If you're going to go to the lengths you mentioned, my suggestion would be to go with 5160.....unlike 4140/42, you can make some minor heat treat errors, and still achieve a very good blade.
 
I agree with Ed about the 5160, but cast 4140 tomahawk/axe heads hold up surprising well in general use. Can't see it as a good knife steel though.
 
I appreciate the insight guys, I have two hawks in 4140 and while they dont hold 3v style edges they are more than adequate for axe uses. I have left the edges around 20-25 degrees a side micro bevel with about a 5 degree a side main bevel and they perform better than my fiskars hatchets on par with my wetterlings. Not as fine of edge but they dont chip as easily either.

I was under the impression 56 was the sweet spot for chopping tools and 4140 achieves this, as well as being known to be incredibly tough.

I cant find 5160 in the sizes I need, other options would be s7, L6, A2, 1075, 1084.
 
I think you will just have to try it and see how well it works for your needs. The chart I saw said to temper at 400° minimum and that would give about a 55 Rc hardness with decent toughness. Your heat for hardening temp with 4140 should be at least 2 or 3 shades of red above non-magnetic. Non-magnetic is a constant 1414°F. Your target heat for 4140 should be 1500° to 1550°, ideally best with a soak if possible, and an oil quench. A warmed vege cooking oil or mineral oil should do. ATF would likely work well enough also if that's the best you can do.
 
You can get 5160 from The New Jersey Steel Baron and Admiral Steel. Of the ones that you mentioned L6, 1075, and 1084 are a lot better than 4140. The S7 and A2 are both air hardening steels and, as such, not really suited to forging though if you know what you're doing it can be done but I see no reason to go to that effort when there are so many steels much better suited to forging.

Doug
 
Thanks guys, Doug not looking to forge, just do my own heat treat after stock removal.
 
If you're using a torch, you may have to do lots of trial and error. The austenizing temperature for 4140 will be considerably higher than for normal blade steels. Undershooting the austenizing temperature will leave you with soft ferrite in the edge, which no amount of tempering later will remove. Most of the alloying elements in 4140 raise the austenizing temperature, as does the lower carbon content.

It would be interesting to see how low an angle you can go before having edge failure in a hatchet/hawk/axe design. I have a chopping knife I use for wood work in the yard and it will still shave my arm after an afternoon of breaking down brush up to 3". It has an edge angle of about 17-20 degrees per side, and is in the 55 HRc range. That said, it will not have the impact of a hatchet/hawk, so I can go thinner without failure.
 
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